1. Field of the Invention
The present invention relates to a radiation image detector constructed to receive radiation representing a radiation image to record the radiation image therein, and to be scanned with readout light to output signals in proportion to the radiation image recorded therein.
2. Description of the Related Art
Various types of radiation image detectors are proposed and put into practical use in the medical and other industrial fields. These detectors generate electric charges when irradiated with radiation transmitted through a subject and record a radiation image of the subject by storing the electric charges.
One of such radiation image detectors is proposed, for example, in U.S. Pat. No. 6,770,901. The detector is constituted by a layer structure that includes the following layers in the order listed below: a first electrode layer that transmits radiation; a recording photoconductive layer that generates electric charges when irradiated with radiation; a charge transport layer that acts as an insulator against the latent image charges and as a conductor for the electric charges of opposite polarity to the latent image charges; a readout photoconductive layer that generates electric charges when irradiated with readout light; and a second electrode layer that includes linearly extending transparent line electrodes that transmit readout light and linearly extending opaque line electrodes that block the readout light disposed alternately in parallel with each other.
When recording a radiation image using the radiation image detector formed in the manner as described above, radiation transmitted through a subject is irradiated on the detector from the side of the first electrode layer with a negative high voltage being applied to the first electrode layer. The radiation irradiated on the detector in the manner described above is transmitted through the first electrode layer and irradiated on the recording photoconductive layer. Then, electric charge pairs are generated in the area of the recording photoconductive layer where the radiation is irradiated, and the positive charges of the electric charge pairs move to the negatively charged first electrode layer, where they combine with the negative charges charged thereon and disappear. In the mean time, the negative charges of the electric charge pairs generated in the manner as described above move toward the positively charged second electrode layer, but they are blocked by the charge transport layer since it acts as an insulator against negative charges and accumulated at the interface between the recording photoconductive layer and the charge transport layer called storage section. This accumulation of the negative charges in the storage section constitutes the recording of the radiation image.
When reading out the radiation image recorded in the manner as described above, readout light is irradiated on the detector from the side of the second electrode layer. The readout light irradiated on the detector is transmitted through the transparent line electrodes of the second electrode layer and irradiated on the readout photoconductive layer, whereby electric charge pairs are generated in the readout photoconductive layer. The positive charges of the electric charge pairs generated in the readout photoconductive layer combine with the negative charges stored in the storage section, while the negative charges of the electric charge pairs combine with the positive charges charged on the transparent line electrodes, whereby electric currents are detected by current detection amplifiers connected to the transparent line electrodes, which are then converted to voltage values and outputted as image signals.
According to the radiation image detector having opaque electrodes that block the readout light, the radiation irradiated toward the area of the readout photoconductive layer corresponding to each of the opaque line electrodes is blocked when the image is read out. This prevents the discharge of the electric charges between the storage section having the latent image stored therein and the opaque line electrodes, so that the amount of electric charges in the area of the readout photoconductive layer adjacent to each of the transparent line electrodes may relatively be increased compared with the case where no such opaque line electrode is provided. Thus, the amount of signal charges obtainable from the radiation image detector to outside through the transparent line electrodes may relatively be increased compared with the case where no such opaque line electrode is provided, whereby readout efficiency is improved.
Here, Japanese Unexamined Patent Publication No. 2003-031836 discloses one of the methods for forming the opaque line electrodes. In the method, an opaque film 45c that blocks readout light is provided on the side of each of the transparent line electrodes 45b from which the readout light is irradiated as shown in FIG. 15A. The radiation image detector 40 shown in FIG. 15A is constituted by a layer structure that includes the following layers in the order listed below: a first electrode layer 41, a recording photoconductive layer 42, a charge transport layer 43, a readout photoconductive layer 44, a second electrode layer 45, and a glass substrate 47. The second electrode layer 45 includes transparent line electrodes 45a that transmit readout light, and opaque line electrodes, each constituted by a line electrode 45b that transmits the readout light and an opaque film 45c that blocks the readout light. An insulation layer 48 is provided between the transparent line electrodes 45a line electrodes 45b and the opaque films 45c. 
In the radiation image detector 40 according to the Japanese Unexamined Patent Publication No. 2003-031836, each of the opaque film 45c is provided such that it covers the region corresponding to the area of the readout photoconductive layer 44 between the side faces of two adjacent transparent line electrodes 45a as shown in FIG. 15A, so that electric charges in the area of the readout photoconductive layer 44 adjacent to each of the transparent line electrodes 45a are not fully discharged, causing degradation in the readout efficiency by that much.
Further, Japanese Unexamined Patent Publication No. 2003-218335 proposes a method for forming the opaque line electrodes by providing opaque films 55c or 65c on the side of the transparent line electrodes 55b or 65b from which readout light is irradiated as shown in FIG. 15B or 15C.
In the radiation image detector 50 or 60 according to the unexamined patent publication described above, each opaque film 55c or 65c is provided such that it extends only to the side edges of each line electrode 55b or 65b as shown in FIG. 15B or 15C, so that the electric charges in the area of the readout photoconductive layer 44 adjacent to each of the line electrodes 55b or 65b are discharged, causing degradation in the readout efficiency by that much. The radiation image detector 50 shown in FIG. 15B is constituted by a layer structure that includes the following layers in the order listed below: a first electrode layer 51, a recording photoconductive layer 52, a charge transport layer 53, a readout photoconductive layer 54, a second electrode layer 55, and a glass substrate 57. The second electrode layer 55 includes transparent line electrodes 55a that transmit readout light and opaque line electrodes, each constituted by a line electrode 55b that transmits readout light and an opaque film 55c that blocks readout light. The radiation image detector 60 shown in FIG. 15C is constituted by a layer structure that includes the following layers in the order listed below: a first electrode layer 61, a recording photoconductive layer 62, a charge transport layer 63, a readout photoconductive layer 64, a second electrode layer 65, and a glass substrate 67. The second electrode layer 65 includes transparent line electrodes 65a that transmit readout light and opaque line electrodes, each constituted by a line electrode 65b that transmits the readout light and an opaque film 65c that blocks readout light. Further an insulation layer 68 is provided between the transparent line electrodes 65a and opaque line electrodes.
In the mean time, in the radiation image detectors described above, residual charges remaining in the storage section are erased after the image is read out in the manner as described above. More specifically, erasing light is irradiated on the detector from the side of the second electrode layer, which causes electric charge pairs to be generated in the readout photoconductive layer, and charges of either polarity combine with the residual charges remaining in the storage section, whereby the residual charges are erased.
However, for example, as in the radiation image detector 60 shown in FIG. 15C, if the insulation layer 68 is provided on the surface of the opaque line electrodes, the electric charges of the other polarity of the electric charge pairs generated by the erasing light in the manner as described above remain on the insulation layer 68 as shown in FIG. 15C. These charges remaining on the insulation layer 68 appear as a residual image on the image obtained in the next radiation image recording and reading out process.
The present invention has been developed in view of the circumstances described above, and it is an object of the present invention to provide a radiation image detector including opaque line electrodes that block readout light with improved readout efficiency.
It is a further object of the present invention to provide a radiation image detector capable of preventing the residual image described above, as well as improving the readout efficiency.